1. Field of the Invention
The present invention relates to a magnetic display erasing apparatus for a magnetic display sheet which is sensitive to a magnetic field and displays visible information.
2. Description of the Related Art
Conventionally, magnetic cards in which a magnetic stripe is formed along the longitudinal direction thereof are widely used in various fields such as bank cards, credit cards, and ID cards. In such cards a magnetic stripe is recorded, for example, to store an identification code of a user of the card, as patterns of orientations or strengths of magnetization.
However, since data recorded in such a magnetic stripe are invisible information, a special apparatus such as a card reader is necessary when the recorded data are to be directly checked.
A magnetic card of another type is known in which a magnetic display sheet, which contains a micro capsule sensitive to a magnetic field, is formed at a position other than the position of a magnetic stripe (for example, Japanese Unexamined Patent Publication JP-A 6-168369 (1994)).
FIG. 12 is a sectional view showing an example of a magnetic display sheet 1. In a capsule 3, flat magnetic flakes 5 are encapsulated together with an oil 4, and a number of capsules 3 are sealed between a substrate member 2 and a protective layer 6. When a magnetic field perpendicular to the magnetic display sheet 1 is applied to the magnetic flakes 5, the magnetic flakes 5 are perpendicularly oriented, so that the optical reflectivity is reduced with the result that the magnetic display sheet 1 becomes dark. On the other hand, when a magnetic field parallel to the card substrate is applied to the magnetic flakes 5, the magnetic flakes 5 are oriented in parallel to each other, so that the optical reflectivity is increased with the result that the magnetic display sheet becomes bright. On this principle, the magnetic display sheet 1 can display visible information such as characters and symbols in accordance with the direction of the applied magnetic field. Consequently, the magnetic card is remarkably improved in ease of use.
FIGS. 13A and 13B are a plan view and a side view, respectively, showing an example of a prior art magnetic display erasing apparatus. Such a configuration is disclosed in Japanese Unexamined Utility Model Publication JP-U 7-6901 (1995), and Japanese Examined Patent Publication JP-B2 54-29895 (1979). A pair of permanent magnets 7 are disposed at a predetermined interval so that the magnetic poles thereof, which confront each other via the magnetic display sheet 1 provided on a surface of the card, have the same polarity, and an erasing operation is realized by moving the magnetic display sheet 1 between the pair of permanent magnets.
In this configuration, a substantially planar magnetic field can be applied to the magnetic display sheet 1, and hence it is expected that the erasing quality is improved.
As shown in FIGS. 13A-13B, however, since the magnets 7, which have an identical plate-like shape (herein round shape) and the same coercive force, are disposed in parallel to each other, at the center of gravity A of a pattern (the center of a circle), which is obtained by projection of the magnets 7 on the plane of the magnetic display sheet 1 (in the example, the pattern is identical with that obtained by orthogonal projection of the magnets 7), the magnetic fields produced by the magnets 7 perpendicularly act on the magnetic display sheet, and cancel each other because the magnetic fields on the center A are equal to each other, with the result that the magnetic field strength on the center A becomes zero.
Consequently, the region of the magnetic display sheet 1 which passes the center of gravity A is not subjected to the erasing operation, so that the magnetic display in this region is not erased and remains as a black line. This is applicable also to the case where the magnets are of a triangular, square, or semicircular shape. When there are plural pairs of poles confronting each other, a black line remains at the center of gravity of each of plural pole region patterns which are obtained by projecting (orthogonally projecting) the regions of the confronting poles on the plane of the magnetic display sheet 1. An ideal planar magnetic field acts on the region other than the vicinity of the center of gravity A, and hence a perfect erasing operation is executed in such a region.
In order to avoid such a region which is beyond the effect of the erasing operation, the magnets 7 must be disposed so that the magnetic display sheet 1 does not pass the vicinity of the center of gravity A. When circular magnets are used as shown in FIGS. 13A-13B, therefore, only a half or less portion of the magnetic fields produced by the magnets 7 is used in the erasing operation, thereby lowering the space utilization efficiency of the magnets 7. As a result, this makes the miniaturization of the whole apparatus difficult and causes further restrictions on the design.